Printing is one of the most popular ways of conveying information to members of the general public. Digital printing using dot matrix is printers allows rapid printing of text and graphics stored on computing devices such as personal computers. These printing methods allow rapid conversion of ideas and concepts to a printed product at an economic price without time consuming and specialised production of intermediate printing plates such as lithographic plates. The development of digital printing methods has made printing an economic reality for the average person even in the home environment.
Conventional methods of dot matrix printing often involve the use of a printing head, e.g. an ink jet printing head, with a plurality of marking elements, e.g. ink jet nozzles. The marking elements transfer a marking material, e.g. ink or resin, from the printing head to a printing medium, e.g. paper or plastic. The printing may be monochrome, e.g. black, or multi-coloured, e.g. full colour printing using a CMY (cyan, magenta, yellow, black=a process black made up of a combination of C, M, Y), a CMYK (cyan, magenta, yellow, black), or a specialised colour scheme, (e.g. CMYK plus one or more additional spot or specialised colours). To print a printing medium such as paper or plastic, the marking elements are used or “fired” in a specific order while the printing medium is moved relative to the printing head. Each time a marking element is fired, marking material, e.g. ink, is transferred to the printing medium by a method depending on the printing technology used. Typically, in one form of printing, the head will be moved relative to the printing medium to produce a so-called raster line which extends in a first direction, e.g. across a page. The first direction is sometimes called the “fast scan” direction. A raster line comprises a series of dots delivered onto the printing medium by the marking elements of the printing head. The printing medium is moved, usually intermittently, in a second direction perpendicular to the first direction. The second direction is often called the slow scan direction.
The combination of printing raster lines and moving the printing medium relative to the printing head results in a series of parallel raster lines, which are usually closely spaced. Seen from a distance, the human eye perceives a complete image and does not resolve the image into individual dots provided these dots are close enough together. Closely spaced dots of different colours are not distinguishable individually but give the impression of colours determined by the amount or intensity of the three colours cyan, magenta and yellow which have been applied.
In order to improve the veracity of printing, e.g. of a straight line, it is preferred that the distance between dots of the dot matrix is small, that is the printing has a high resolution. Although it cannot be said that high resolution always means good printing, it is true that a minimum resolution is necessary for high quality printing. A small dot spacing in the slow scan direction means a small distance between marker elements on the head, whereas regularly spaced dots at a small distance in the fast scan direction places constraints on the quality of the drives used to move the printing head relative to the printing medium in the fast scan direction.
Generally, there is a mechanism for positioning a marker element in a proper location over the printing medium before it is fired. Usually, such a drive mechanism is controlled by a microprocessor, a programmable digital device such as a PAL, a PLA, a FPGA or a similar device although the skilled person will appreciate that anything controlled by software can also be controlled by dedicated hardware and that software is only one implementation strategy.
One general problem of dot matrix printing is the formation of artefacts caused by the digital nature of the image representation and the use of equally spaced dots. Certain artefacts such as Moiré patterns may be generated due to the fact that the printing attempts to portray a continuous image by a matrix or pattern of (almost) equally spaced dots. One source of artefacts can be errors in the placing of dots caused by a variety of manufacturing defects such as the location of the marker elements in the head or systematic errors in the movement of the printing head relative to the printing medium. In particular, if one marking element is misplaced or its firing direction deviates from the intended direction, the resulting printing will show a defect which can run throughout the printing. A variation in drop velocity will also cause artefacts when the printing head is moving, as time of flight of the drop will vary with variation in the velocity. Similarly, a systematic error in the way the printing medium is moved relative to the printing medium may result in defects that may be visible. For example, slip between the drive for the printing medium and the printing medium itself will introduce errors. In fact, any geometrical limitation of the printing system can be a source of errors, e.g. the length of the printing head, the spacing between marking elements, the indexing distance of the printing medium relative to the head in the slow scan direction. Such errors may result in “banding” that is the distinct impression that the printing has been applied in a series of bands. The errors involved can be very small—the colour discrimination, resolution and pattern recognition of the human eye are so well developed that it takes remarkably little for errors to become visible.
To alleviate some of these errors it is known to alternate or vary the use of marking elements so as to spread errors throughout the printing so that at least some systematic errors will then be disguised. For example, one method often called “shingling” is known from U.S. Pat. No. 4,967,203, which describes an ink jet printer and method. Each printing location or “pixel” can be printed by four dots, one each for cyan, magenta, yellow and black. Adjacent pixels on a raster line are not printed by the same marking element in the printing head. Instead, every other pixel is printed using the same marking element. In the known system the pixels are printed in a checkerboard pattern, that is, as the head traverses in the fast scan direction a marking element is able to print at only every other pixel location. Thus, any marking element that prints consistently in error does not result in a line of pixels in the slow scan direction, each of which has the same error. However the result is that only 50% of the marking elements in the head can print at any one time. In fact, in practice, each marking element prints at a location that deviates a certain amount from the correct position for this marking element. The use of shingling can distribute these errors through the printing. It is generally accepted that shingling is an inefficient method of printing as not all the marking elements are used continuously and several passes are necessary.
As said above, this kind of printing has been called “shingling”. However, printing dictionaries refer to “shingling” as a method to compensate for creep in book-making. The inventors are not aware of any industrially accepted term for the printing method wherein no adjacent pixels on a raster line are printed by one and the same marking element. Therefore, from here on and in what follows, the terms “mutually interstitial printing” or “interstitial mutually interspersed printing” are used. It is meant by these terms that an image to be printed is split up in a set of sub-images, each sub-image comprising printed parts and spaces, and wherein at least a part of the spaces in one printed sub-image form a location for the printed parts of another sub-image, and vice versa.
Another method of printing is known as “interlacing”, e.g. as described in U.S. Pat. No. 4,198,642. The purpose of this type of printing is to increase the resolution of the printing device. That is, although the spacing between marking elements on the printing head along the slow scan direction is a certain distance X, the distance between printed dots in the slow scan direction is less than this distance. The relative movement between the printing medium and the printing head is indexed by a distance given by the distance X divided by an integer.
There is a continuous requirement for improvements in printing quality. In particular, there is a requirement that dots forming the images are properly placed and of uniform size, and wherein the method of formation of the dots is resistant to degradation with extended use of the print head.
It is known from U.S. Pat. No. 4,963,882 to use an ink jet printer that tries to solve the above problem. The solution described uses a double dotting approach, whereby each dot is formed from at least two droplets of each colour, ejected from different marking elements. It is a disadvantage of the solution described that, if in a print head a marking element breaks down, and it happens to be a marking element which compensates for a marking element which prints badly (e.g. with a deviation between the desired printing location and the actual printing location), then either the error becomes visible and image quality is deteriorated, or the print head has to be replaced. If two marking elements are defect, which are exactly the marking elements which should be printing on the same dot location, then a blank printing result is obtained, and in that case the print head should definitely be replaced.
U.S. Pat. No. 6,126,341 describes a print head with a plurality of printing marking elements. When one of the plurality of marking elements is defective, a control device reads out the image data for the defective marking element from a first image data storing device, and saves the readout image data into a second image data storing device. After a swath has been printed by the normal marking elements, while the defective marking element is not driven, a substitution marking element is selected from among the normal marking elements or from a set of substitution marking elements, and is caused to print on a location on the sheet which would otherwise be subjected to printing by the defective marking element, in accordance with the image data read out from the second image data storing device. Therefore an extra printing pass is needed in order to back-up the defect marking element, and thus printing slows down. It is an object of the present invention to provide a printing method and apparatus which provide high resolution printing at high speed with a reduced visible effect of errors.